The most commonly used solution for producing such a receiver is that of providing a photodiode with a TIA (an acronym for the English expression “Transimpedance Amplifier”); the sensitivity performance is dependent on this TIA, which has a high gain bandwidth product (or “GBW”, an acronym for the English expression “Gain Bandwidth Product”) and very low noise.
A photodiode 1 is conventionally represented by the circuit shown in FIG. 1a. As shown in the left-hand figure, this photodiode is preferably charged by a resistor R′d between the anode and the ground so as to absorb the direct current due to the ambient illumination, also called the background current, which is included in the light signal received by the photodiode. According to the equivalent representation shown in the right-hand figure, this resistor R′d in parallel with the internal resistance of the photodiode forms an equivalent resistor Rd. The photodiode is generally characterized by a capacitance Cd between the anode and the ground, shown in the right-hand figure.
In a conventional receiver circuit, an example of which is shown in FIG. 1b, a photodiode 1 of this type is associated with a TIA 2 via a linking capacitor Cliaison which helps to separate the useful pulses from the background current. The value of this linking capacitor Cliaison is typically more than 10 nF. Let us recall that a TIA comprises, in parallel, an operational amplifier AOP or an amplifier with discrete components, a feedback resistor Rf and a stabilizing capacitor Cf. Such a receiver makes it possible to neutralize the effect of the parasitic capacitance Cd of the photodiode by means of a virtual ground.
To a first approximation, this is a second-order loop system:                Having a conversion gain ZT(p) such that        
                                          Z            T                    ⁡                      (            p            )                          =                                            V              s                                      i              D                                =                                    -                              R                f                                      ⁢                          1                              1                +                                                                            2                      ⁢                                                                                          ⁢                      ζ                                                              ω                      n                                                        ⁢                  p                                +                                                      p                    2                                                        ω                    n                    2                                                                                                          (                  eq          ⁢                                          ⁢          1                )                            where Vs is the output voltage of the circuit, iD is the current generated by the photodiode, p (p=jω=j2πf) is the Laplace variable, Rf is the feedback resistance of the TIA, and ξ is the damping of the receiver,        and having a natural frequency ωn such that:        
                              ω          n                =                                            2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              GBW                                                      R                f                            ⁡                              (                                                      C                    d                                    +                                      C                    f                                                  )                                                                        (                  eq          ⁢                                          ⁢          2                )            
The ratio of damping to natural frequency can be written thus:
                              ζ                      ω            n                          =                              1            2                    ⁢                      (                                                            R                  f                                ⁢                                  C                  f                                            +                                                1                                      2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    GBW                                                  ⁢                                  (                                      1                    +                                                                  R                        f                                                                    R                        d                                                                              )                                                      )                                              (                  eq          ⁢                                          ⁢          3                )                            or in practice:        
                                          R            f                    ⁢                      C            f                          >>                              1                          2              ⁢              π              ⁢                                                          ⁢              G              ⁢                                                          ⁢              B              ⁢                                                          ⁢              W                                ⁢                      (                          1              +                                                R                  f                                                  R                  d                                                      )                                              (                  eq          ⁢                                          ⁢          4                )                            this ratio then takes the simple form:        
                              ζ                      ω            n                          ≈                              1            2                    ⁢                      R            f                    ⁢                      C            f                                              (                  eq          ⁢                                          ⁢          5                )            
The gain modification is found according to Equation (1) from the change in the value Rf which, according to Equation (2), modifies the natural frequency ωn and hence the damping ξ according to Equation (5). With a conventional solution, therefore, it appears to be difficult to change the gain without modifying the transfer function.
The frequency response is shown in FIG. 5a for three damping values ξ (0.9, 0.7 and 0.5). This figure demonstrates that the change in gain affects the damping when the band is kept constant.
Another important criterion is the equivalent current noise applied to the input of the TIA, which is written thus:
                              i          n                =                                            i                              n                -                            2                        +                                          (                                                      e                    n                                                        R                    f                                                  )                            2                        +                                          4                ⁢                kT                                            R                f                                                                        (                  eq          ⁢                                          ⁢          6                )            where in− and en, respectively, are the equivalent noise current at the negative input of the operational amplifier AOP and the equivalent noise voltage at the input of AOP which characterize the operational amplifier used, k is the Boltzmann constant, and T is the temperature in degrees Kelvin.
For a given TIA and a given photodiode, the sensitivity is optimized by choosing the highest possible resistance Rf compatible with the pulse processing band.
However, as the gain increases, the admittance decreases, because the voltage range at the output of the amplifier is fixed by the power supplies. Conversely, a decrease in gain increases the admittance but degrades the noise, with a current limitation determined by the maximum output current of the amplifier.
The problem therefore arises of providing an optimum receiver for weak signals but also for strong signals, while preferably maintaining the same frequency response. The conventional solutions are:                Reducing the gain of the TIA by reducing the feedback resistor Rf which determines the conversion gain of the TIA, thereby improving the admittance but worsening the noise. Furthermore, reducing the feedback resistance has the effect of significantly increasing the bandwidth, which is evidently undesirable if a pulse shape independent of gain is required.        Placing a switched resistive attenuator between the photodiode and the TIA so as to reduce the gain when the received level exceeds the admittance. This degrades the noise, because the resistances generate noise. Moreover, the switches have non-negligible parasitic capacitance relative to the capacitance of the photodiode, which affects the transfer function.        
The conventional solutions do not meet the requirement.
Consequently there is still a need for a receiver with a wide dynamic range, optimized in terms of noise.